Proper adjuvant use boosts herbicide effectiveness

By Larry Stalcup

Tight supplies, inflation, and tough weeds nearly guarantee higher 2023 herbicide costs. One way to boost herbicide performance, though, is through properly mixed adjuvants.

Numerous products fall under the adjuvant banner, but most can be grouped into surfactants, crop oil concentrates, and ammonia fertilizers that accompany herbicides into a spray tank. Adjuvants also include drift reduction agents (DRAs) that curb herbicide off-target movement and pH modifiers that reduce tank mix acidity.  

Adjuvants work by enabling herbicides to get into, onto, and through a weed. ″We must gain every advantage we can to control the target weeds. That’s where the optimization with adjuvants is critical,″ says Bryan Young, a Purdue University weed scientist.

That’s because herbicide costs are climbing. ″A typical Indiana corn herbicide program cost $35 to $45 per acre in 2022, not including application costs,″ Young says. ″Cost for a soybean herbicide program was $40 to $50 per acre. Some soybean farmers were spending $50 per acre just on postemergence weed control. That’s with no preemerge or soil residual program of Enlist One, Roundup, and Liberty.″

In comparison, quality adjuvants can cost $3 to $6 per acre. ″Return on investment for adjuvants can be one to four times the cost,″ he says.

However, adjuvant management is key. If coverage rates lag because of poor adjuvant management, Young says a ″rescue″ post application will cost at least $20 per acre.

MATCHING ADJUVANTS TO HERBICIDE

Public and company crop protectant consultants can help growers select the right adjuvant for the job. ″They look at how adjuvants vary by product, herbicide site-of-action, weed species and usually have lots of research to test and validate their recommendations,″ says Gail Stratman, FMC regional technical service manager.

Having the right adjuvant in the tank ″will ensure every droplet sprayed is as effective as possible,″ says Connor Ferguson, Wilbur-Ellis branded technologies data manager. ″With contact herbicides, adjuvants help droplets adhere to the leaf surface, increasing contact area for better absorption and efficacy. With systemic herbicides, which move within the plant, it’s important to have the right adjuvant to get spray onto the plant leaves so the herbicide can work effectively. This can be difficult with a heavy weed infestation, waxy-type leaf surfaces, or when targeting an otherwise difficult weed like common lambsquarter. In these cases, oil-based adjuvants can help droplets penetrate tough plant surfaces.″

Herbicide advisers do a good job of identifying and approving adjuvants that can help mitigate off-target movement, including synthetic auxin herbicides that include dicamba and 2,4-D, says James Reiss, Precision Laboratories senior vice president of product development.

″But the real opportunity to maximize overall performance comes from understanding the adjuvant needs of the herbicide tank mix partner that is being added to that auxin application,″ he adds.

High surfactant methylated seed oils (HSMSO) or even high surfactant oil concentrates (HSOC) can drive the performance of the tank mix partner and even improve the auxin herbicide’s performance, he says.

MIXING TIPS

The biggest mistakes growers make in using adjuvants is not following the correct mixing order when formulating them in tank mixes,″ says Connor Ferguson, Wilbur-Ellis branded technologies data manager.

He recommends using the ″WALES and DALES″ method to help avoid mixing mess-ups.

He defines them with this order of mixing:

• W/D: Dry formulations such as wettable powders, water dispersible granules, water-soluble packets or dry flowables are added first to sprayer tank water.
  

• A: Agitate thoroughly to ensure proper mixing.
  

• L: Liquid flowables and suspension concentrates are next.
  

• E: Emulsifiable concentrate formulations are next.
  

• S: Surfactants are added last.

″Many products also need time to agitate, thoroughly dissolve, and mix within the tank before application,″ Ferguson says.

MIXING MISTAKES

″Probably the biggest misstep by farmers is beginning the mixing process before the sprayer is at least half full of water,″ says Kyle Gustafson, WinField United crop protection product manager. ″Many crop protection products are formulated as a concentrate, so they need water to dilute. When you start putting too many products in the tank too early in the mixing process, they ‘fight’ for the free water and can react with each other, causing compatibility problems.″

Gustafson says that dry products must be fully soluble in water before adding more products.

″You don’t want to add a dry product into the mixture, then immediately add another liquid product that contains oil that may cause the dry product not to suspend properly,″ he says. ″Always reference proper mixing order before you begin mixing up a load of products.″

Stratman says most product labels specify herbicide should be added to the tank first to ensure it is properly mixed or dissolved, especially in the case of drift function products.

″Many times, growers want to have their adjuvants already premixed in their spray solution for convenience or to ‘condition’ the water, particularly with solutions like UAN or AMS fertilizers,″ he says. ″That may work sometimes, but many times it causes problems getting suspension or dry-flowable formulations to disperse properly. This can result in a variety of problems that can range from clogging nozzles to uneven application, which in turn result in crop injury or weed control issues.″

Young adds that botched mixing may occur when growers or other applicators fail to consult with retailers or others.

″Not all product combinations and mixing scenarios can be covered by a herbicide or adjuvant label,″ he says. ″Retailers and custom applicators have more experience with products that are popular in your local area and have likely learned from previous mistakes.″

OFF-TARGET DRIFT REDUCTION POTENTIAL

Besides new nozzle configurations and application techniques, adjuvants can help manage the off-target potential of synthetic auxin herbicides such as dicamba and 2,4-D. Adjuvants can reduce off-target potential by reducing the relative amount of driftable fine spray droplets being produced during the application, says Young.

″Some adjuvants include retention aids that can reduce the bounce and shatter of these large spray droplets at impact and reduce droplet runoff on more vertical leaf surfaces like grass weeds,″ he says.

Young notes that while adjuvants can reduce the risk of spray drift, they ″will not eliminate″ the chance for drift, especially when poor application decisions are made regarding:

• Nozzle design
  

• Tip size
  

• Spray pressure
  

• Proximity to sensitive plants

• Wind direction
  

• Excessive wind speeds
  

Stratman points out that getting the most out of adjuvant with drift reduction agents also involves using proper boom height and humidity levels. The adjuvant is one piece in the toolbox that can help, but all the other aspects of spray drift management are equally as important, he says.

Gustafson reminds growers that nozzle selection depends on the chemistries being used. ″Low-volatility dicamba and 2,4-D formulations have specific nozzle specifications as required by the label,″ he says. ″Glufosinate requires fine, small droplets, while other postemergence corn herbicides accommodate larger droplet sizes. Be sure you have appropriate nozzles stocked for each herbicide you plan to spray to optimize application efficacy.

″For contact-type chemistries, coverage is essential, which means higher carrier volumes and smaller droplet sizes are needed, he adds. ″For systemic chemistries, coverage is less critical, so nozzles with lower carrier volumes and larger droplets can be used.″

CPDA CERTIFICATION IS KEY

The market offers 2,000 or more adjuvants. Many states don’t regulate them. However, the Council of Producers and Distributors of Agrotechnology (CPDA) monitors adjuvant effectiveness.

Adjuvants that meet CPDA standards are considered CPDA Certified Adjuvants. There are more than 225 CPDA-certified adjuvants on 500-plus crop protection product labels, says James Reiss, Precision Laboratories senior vice president of product development.

″When users purchase a CPDA-certified adjuvant product, they have the assurance that the product meets the functionality claims indicated on the label,″ he says.

Reiss adds that CPDA is launching the first-ever performance-based certification program to help growers make better decisions when matching pesticide formulations with nozzles and adjuvants. That and other helpful information can be the recipe to help prevent the need to repeat post applications.

Read more at Successful Farming.